P3.3 Keeping Things Moving

Question 1

Give two ways you would induce P.D

Answer 1

• Moving a magnet in a coil of wire.

- Moving an electrical conductor in a magnetic field.

Question 2

How do you make an A.C using induction?

Answer 2

1) Turn a magnet end to end in a coil - the magnetic field will change - this induces P.D - current will flow in the wire.
2) When you’ve turned the magnet through 1/2 turn, the direction of the magnetic field through the coil reverses and so current will flow in the opposite direction around the wire.
3) Keep turning the magnet in the same direction = AC current

Question 3

What is the relationship between magnets and potential difference?

Answer 3

If you move a magnet in the opposite direction/reverse polarity = potential difference/current = reversed.

Question 4

What is the function of a dynamo?

Answer 4

Used on top of bikes to turn on lights

Question 5

How does a dynamo work to power its lights?

Answer 5

• Cog wheel at the top positioned to touch one of the wheels.
• As wheel turns, turns cog wheel which is attached to a magnet.
• Creates an AC to power the lights.

Question 6

Define the motor effect

Answer 6

The force experienced by a current carrying wire in a magnetic field.

Question 7

How is the motor effect force made bigger?

Answer 7

• Stronger magnetic field

- More current

Question 8

When is full force experienced in a motor?

Answer 8

When the wire is 90 degrees to the magnetic field.

Question 9

When is no force experienced in a motor?

Answer 9

When the wire is parallel to the magnetic field.

Question 10

When is some force experienced in a motor?

Answer 10

When the wire is at an angle other than 90 degrees to the magnetic field.

Question 11

What is the relationship between the direction of force and the magnetic field?

Answer 11

The direction of force is reversed if the magnetic field is reversed.

Question 12

What does a split-ring commutator do?

Answer 12

Swaps the contracts every 1/2 turn to keep the motor rotating in the same direction.

Question 13

How is the motor direction reversed?

Answer 13

• Swapping the polarity of the DC

- Swapping the magnetic poles over

Question 14

Where are motors used?

Answer 14

• Fans
• CD players
• Food mixers
• Hair dryers

Question 15

What is induction?

Answer 15

The creation of a potential difference across the ends of an electrical conductor which is experiencing a change in magnetic field.

Question 16

What are transformers made up of?

Answer 16

• Magnetically “soft” iron core
• Primary coil covered in plastic
• Secondary coil covered in plastic

Question 17

What does a step up transformer do and how do its features show this?

Answer 17

It steps the voltage up and has more secondary coil then primary coil.

Question 18

What does a step down transformer do and how do its features show this?

Answer 18

It steps the voltage down and has more primary coils than secondary.

Question 19

How does a transformer work? (Think induction)

Answer 19

• AC in primary coil means magnetic field in iron core constantly changing direction.
• Rapidly changing magnetic field felt by secondary coil.
• Changing field induced an alternating P.D across secondary coil.
• Relative number of turns on 2 coils determines whether greater or less.

Question 20

Why do transformers only work in A.C?

Answer 20

• There would still be a magnetic field in the iron core but it wouldn’t be constantly changing.
• So no induction in the secondary coil as you need a changing field to induce P.D

Question 21

What frequencies do switch mode transformers work at?

Answer 21

50KHz - 200KHz

Question 22

What are the advantages of switch mode transformers over traditional transformers? (2)

Answer 22

• They work at higher frequencies so they are lighter and smaller (more useful for mobile chargers etc)
• More efficient as they use little power when switched on but no load is applied.

Question 23

What is the iron core for in a transformer?

Answer 23

It transfers the changing magnetic field from the primary coil to the secondary coil.
It carries NO current.

Question 24

What is a magnetic field?

Answer 24

A region where magnetic materials and wires carrying currents experience a force.

Question 25

How is an electromagnet created?

Answer 25

• When a current flows through a wire/coil, a magnetic field is created around it.
• Magnetic field inside the wire = strong and uniform
• Magnetic field outside= like a normal magnet.

Question 26

How do you strengthen the magnetic field around a coil of wire?

Answer 26

By adding a magnetically “soft” iron core through the middle of the coil.

Question 27

What is a use of electromagnets?

Answer 27

Cranes for lifting rubbish in junk yards

Question 28

What happens if there is a permanent magnet near an electromagnet?

Answer 28

• The magnetic field created by the electromagnet interacts with the permanent magnetic field of the permanent magnet
• This produces a resultant force and the coil moves

Question 29

What is the relationship between the direction of the force and the direction of the current?

Answer 29

When the direction of the current changes direction so the direction of the force changes to the opposite direction